The present invention relates generally to antennas and, more particularly, to small and high efficiency antennas for mobile and handset communication devices.
Mobile communication devices are becoming smaller as the technology is developed. For an antenna to operate properly, it should usually be about half a wavelength in size, except for monopole-like antennas (which normally operate above a ground plane), where a quarter wavelength is required. For advanced mobile communication devices, e.g., cellular handset units, such dimension are impractical since the overall handset dimension is smaller than half a wavelength of the appropriate frequency.
Using small antennas reduces their efficiency, and hence requires higher power to be supplied in order to operate the device. Higher power causes shorter battery cycles between charging and increases the radiation into the user""s head/body. The level of power radiated into the human head is most significant, and serious limitations and specifications are prescribed in order to protect the users.
Operation of such devices adjacent to a human body also changes the field and/or current distribution along the antenna, and hence changes its radiation pattern, as well as the radiation efficiency. Practically speaking, the reduction in efficiency may be even in the range of 10-20 dB or more. The result is a requirement for higher power to operate the device with the consequent disadvantages described above. The use of external whip antennas, such the xe2x80x9cSTUBBYxe2x80x9d or retractable antennas, is also inconvenient, as the antennas are often xe2x80x9ccaught upxe2x80x9d inside the pocket. They also detract from the aesthetic appearance of the mobile communication device and most importantxe2x80x94the radiation pattern is quasi-omni, so no enhancement is achieved in radiation at the user""s head/body.
Internal antennas supplied by several companies are relatively inefficient as compared to external antennas. Furthermore, these known internal antennas generally do not decrease the radiation into the user""s head/body, and in many cases even increases such radiation. The antenna gain is also generally poor (especially while used adjacent to the head/body), and the SAR (Specific Absorption Ratio) results are generally high.
Another problem in the known internal antenna is the narrow bandwidth of operation. In addition to the narrow bandwidth where the input impedance is matched the radiation efficiency is even further reduced. The latter is considered an even more difficult problem in cases where dual frequency bands or triple-band operations of the mobile communication devices are required, such as cellular GSM 900/1800, 900/1900, 900/1800/1900 MHz, etc.
Internal antennas for mobile communication devices are known that utilize a resonant radiation element as the main radiator. In particular, printed antennas, e.g. patches and slots, are very convenient to use because of their ease of manufacture, their low profile, and their low production cost. If such printed elements could be used in mobile communication devices with respect to efficiency, gain, impedance matching and reproducibility, it would be the best choice. Unfortunately, such elements, because of the small size of the mobile communication device, will show very low efficiency and hence low gain, and it will be difficult to match their impedance to that of the mobile communication device.
Generally, slots excited by a feed line (e.g., by microstrip or stripline structures) or by a coax cable, are usually narrow band. In order to achieve matching of the slot even over a narrow band, the excitation of the slot is generally made off-center, to reduce the input impedance of the slot, which is naturally very high. U.S. Pat. No. 5,068,670 by one of the inventors in this application and hereby incorporated by reference describes a broadband slot antenna achieved by adding matching networks at both sides of the slot. In the preferred embodiment, the feed lines are located off-center of the slot.
The direction of maximum radiation of an off-center excited slot is changed with frequency due to the asymmetrical electric and magnetic field distributions excited along the slot. While narrow bandwidth slots are not significantly affected by this phenomenon, broadband slots are indeed affected. The best solution is to excite the slot symmetrically by dual feed and load lines, which may be split from a single excitation feed. Each of the strip arms has a dual matching network in order to widen the bandwidth of the antenna. The length and width of each arm may be equal in order to achieve full symmetrical structure, but may also differ in order to maximize the bandwidth. If the arms art not identical, there will be some squint with frequency.
The slot may be a non-resonant one, by making it open at both ends (xe2x80x9copen-endedxe2x80x9d), or a resonant one, by making it closed at both ends (xe2x80x9cshort-endedxe2x80x9d). The reaction efficiency depends on the field distributionxe2x80x94amplitude and phase, along the slot. The fields in short-ended slots mush vanish at both ends of the slot; and since they are continuous, their value at any point along the slots cannot reach the required level as with shorter slots. Therefore, short-ended slots are relatively large, usually in the range of half wavelength at the operation frequency.
The fields in open-ended slots may have finite value at their ends and should not vanish. It follows that a reasonable value of the field can be reached even for relatively short-length slots. The excitation point may then be optimized for single or dual feeds. It should be taken into consideration that radiation pattern will be different from the usual one. Further, the load type of the strip for open-ended slots would preferably be of the form of a short circuit, to eliminate a floating ground at the far end of the slot. As a result, this configuration is more complex to match by means of the relative part of the slot impedance. Furthermore, a floating ground would decrease the antennas efficiency.
EP 0924797 describes a slot antenna configuration in which the slot is curved along two axes, and is excited at its center point by a coax cable. There are a number of disadvantages of such configuration as suggested by this patent. Thus, the matching of such a slot is very difficult due to the centered excitation point (as described above and in U.S. Pat. No. 5,068,670). In addition, the part of the slot which contributes to the radiation in the desired direction is very small while, due to the folded arms of the slot which are parallel, the fields are opposite in polarization and hence cancel the radiation at most desired directions. Further, the excitation is complex and costly to implement. Finally, slots which are open-ended at one end are less efficient as compared to short-ended slots, and cause radiation in undesired directions. The radiation pattern will be asymmetrical due to the radiation from the open end of the slot, since the fields do not vanish, as above-mentioned.
U.S. Pat. Nos. 5,929,813 and 6,025,802 describe similar antennas. Such antennas are actually loop antennas where a xe2x80x9cwired slotxe2x80x9d generates a loop antenna. There are a number of disadvantages of such configuration as suggested by this patent. Thus, xe2x80x9cwired slotxe2x80x9d is open at the connecting points, is cut along the edge of the antenna and is also folded on the metal sheet, hence it causes radiation in undesired directions and with opposed (horizontal) polarization. The xe2x80x9cwired slotxe2x80x9d is excited by the antenna connector very close to the antenna (and telephone) edge; hence, radiation at the user""s head is not reduced. Actually, because of the phone""s PCB, which significantly contributes to the radiation at CDMA/TDMA/GSM frequencies (800 and 900 MHz), it would appear that the radiation at the user""s head is even increased.
Further, in the embodiment of a dual frequency operation according to these referenced patents, the radiation pattern in the higher band has nulls, or at least significant reduction at certain angles and is far from being omni-directional in the azimuth plane. In this configuration each xe2x80x9cwired slotxe2x80x9d affects the operation of the other band when it is not supposed to influence the loop produced by this configuration is parallel to the user""s head in xe2x80x9ctalk positionxe2x80x9d (e.g. a position where the user holds the mobile communication device adjacent to his head), and hence the fields"" distributions are significantly changed by the human body.
As a result, the performance of the antenna is reduced, high transmitted power level would be required, and the sensitivity of reception would be less than required.
U.S. Pat. No. 6,002,367 describes a slot antenna excited by a feed line, similar to the structure described in U.S. Pat. No. 5,068,670. The slot is excited at its center point, and is very small as compared to the wavelength at the operational frequency; hence it does not radiate efficiently. The patch (or patches) added above the slot is (are) excited by the feed line; the load line (described in several embodiments) and the grounding of the patch tune the patch. This antenna mechanism is similar to that of the well-known Planar Inverted xe2x80x9cFxe2x80x9d Antenna (PIFA), where the grounding of the element tunes the antenna, except for the signal feeding, which is made by a feed line rather than a probe (PIFA). The performance of the antenna is average and less. It is complicated to build and relatively expensive, and no real reduction in the radiation at the user""s head/body is achieved. In addition, the structure""s height is large even in the simplest embodiment of a single patch. For modern mobile communication devices, which are very compact in size, such dimensions are impractical. Other antenna constructions are described in WO 99/13528, and WO 99/36988 (U.S. Pat. No. 5,945,954) but such antennas also suffer from one or more of the drawbacks discussed above.
An object of the invention is to provide an internal antenna for mobile communication devices which, although very small as compared to conventional antennas, yet is nevertheless capable of operating at high efficiency.
Another object of the invention is to provide an internal antenna for mobile communication devices displaying low Specific Absorption Ratio (SAR) with respect to the radiation at the head/body of a human person.
A further object of the invention is to provide an internal antenna for mobile communication devices wherein operation in the vicinity of a human head/body does not significantly interfere with the performance of the antenna.
Another object of the invention is to provide an internal antenna for mobile communication devices that can efficiently operate in wide frequency bandsxe2x80x94single, dual or multi-band.
A further object of the invention is to provide an internal antenna for mobile communication devices that can be manufactured inexpensively in volume as compared to the conventional external antennas.
Yet another object and advantage of the invention is to provide an internal antenna for mobile communication devices that presents a more aesthetic appearance than the comparable devices equipped with conventional external antennas.
According to one aspect of the present invention, there is provided a multi-band microwave antenna which is resonant and radiant at a high frequency band and at least one lower frequency band, comprising: a dielectric substrate having opposed faces; an electrically-conductive layer serving as a ground plane on one face of the dielectric substrate; an electrically conductive feed line carried on the opposite face of the dielectric substrate, the feed line having at least one feed end and at least one load end; a slot formed in the ground plane having a feed side and a load side with respect to the feed end and load end, the slot being electromagnetically coupled to the load end of the feed line such that the slot is resonant and radiant at the high frequency band; and a further electrical conductor electrically connected to the ground plane to serve as a continuation thereof at the load side of the slot, the further electrical conductor being dimensioned, located and electromagnetically coupled to the slot at the lower frequency band such as to cause the slot to be resonant and radiant also at the at least one lower frequency band.
The explanation for the enhancement in the lower operational frequency is as follow: Electrical current are generated along the ground plane of the antenna, which contribute to the radiation of the antenna. In a finite ground plane, these currents generate electric and magnetic fields at both ends of the ground plane (those ends which are perpendicular to the direction of propagation of the currents), acting like a patch antenna. The currents generated along the ground plane must be continuous and therefore, if the size of the ground plane is small, no significant current amplitude will be achieved, (theoretically, around one-half wavelength is required for maximum current to be generated). By adding the second ground plane, the generated currents do not need to vanish at the first ground plane""s edge and thus contribute to the radiation of the slot. The reason for the order of one half wavelength is based on the phase of the current which has a difference of 180xc2x0 at both edges. The generated field at the edges, which are the product of multiplication of the current and the normal to the edge (which is opposed in direction at both edges) yields in-phase electromagnetic fields and hence contribute to the radiation at desired directions.
In order to keep the antenna surface small as usually required for mobile communication devices, the second ground plane may be folded or placed above or below the first ground plane, and then the two layers may be connected by pins or other metal members to achieve this continuation of the ground plane and the generated currents. The latter enables the continuation of the currents without affecting the vicinity of the antenna. This added layer may be located in the gap required between the antenna and the communication device, so the total volume remains the same. This gap is required in order to eliminate cancellation of the electromagnetic field/s due to reflected fields off the mobile communication device""s PCB.
The electrical conductor serving as a continuation of the ground plane may also be in the form of an added stub. Such an implementation of the invention saves the need for an extra layer, simplifying the manufacturing and assembly processes, as well as reducing the antenna cost. Plated-through-holes (PTH), metal pins, pads, or any type of electrical conductive members may connect the ground plane on one side and the added stub on the other side.
The entire antenna may be produced on a single-layer flexible printed circuit board then folded thereby eliminating the need for a separated second layer and special connections thereto. It may also be produced on a single dielectric substrate in which the electrical conductor serving as a continuation of the ground plane is formed on the same face as the feed lines but insulated therefrom.
The width of the electrical contacts controls the operational frequency of the lower band. A narrow connection lowers the operational frequency of the lower band, while a wider connection increases the operation frequency of the lower band. The connection may be of the inductive type to act as a low pass filter, and therefor would hardly affect the upper band.
The connection of the antenna to the mobile communication device can be through conductive pins. Either cylindrical, flat or other cross-section pins can be used. The pins can be spring-loaded pins, rigid pins with elastic elements on either the communication device""s PCB or the antenna, or threaded rigid pins. In another embodiment, conductive pins can be soldered to the communication device.
Another method of connection can be through a coaxial connector. The connection can also be made using a flexible PCB as the substrate of the antenna, which can be directly mounted or connected via connector or through pins to the PCB of the communication device.
In the preferred embodiments of the invention described below, the antenna is of the type described in the above-cited U.S. Pat. No. 5,068,670 (of one of the joint inventors in the present application and incorporated by reference herein), in that it includes an electrically conductive feed line carried on a face of a dielectric substrate opposite to that serving as the ground plane, and a slot formed in the ground plane having a feed side electromagnetically coupled to the feed end of the feed line, and a load side electromagnetically coupled to the load end of the feed line, such that the slot is resonant and radiant at a predetermined high frequency band.
According to another aspect of the present invention, the slot formed in the ground plane of such an antenna is curved.
The enhancement achieved by curving the slot is in reducing the overall size of the antenna board. Especially in the case of a slot with both ends shorted, the effect of curving the slot is minimal regarding performance, since the side arms of such slot are in the neighborhood of the slot""s ends. As described earlier, the electric and magnetic fields in a short ended slot vanish at the end of such slot, and since they must be continuous, it follows that their values near the ends of the slot are low and hence are not effected by curving the slot. The region near the center of such slot is most significant, and the values of the fields are high.
The combination of such curved slot and a distributed feed line (preferably similar on that described in U.S. Pat. No. 5,068,670) provides particularly good results especially with such small antennas.
A typical antenna dimension in a typical DCS/PCS frequencies (1800 and 1900 MHz) should be around 60-80 mm. The size is impractical for modern mobile communication devices, where a typical room for an internal antenna is in the range of only (35-45) mmxc3x97(20-30) mm. Prior art slots used so far, such U.S. Pat. Nos. 5,929,813 and 6,025,802 (by Nokia) are fed directly by pins. Further, the structure suggested by these patents are, in fact, loop antennas rather than slot antennas. Further, the structure suggested by these patents are, in fact, loop antennas rather than slot antennas.
PCT/US99/0085, WO 99/36988 (by Rangestar) presents slot antennas for cellular handsets. This suggested antenna is fed by coax and therefore there is no room for any impedance matching rather than the excitation point position along the slot. This configuration is also complex regarding assembly, since it must be soldered, and the wires of the coax may be often broken. Furthermore, the slot is straight rather than curved and is very small in length as compared to the wavelength at the operating frequency, and hence its efficiency is inherently very poor.
Thus, curving the slot while yet exciting it by a distributed feed line having a feed end (preferably including a transformer effected by changing its length and width in order to match the slot impedance) and a load end (which includes a reactive loadxe2x80x94either an open stub, short stub or lumped elements for mainly reducing the reactive part of the slot impedance to a level of zero) provides particularly good results when curving the slot, and exciting it by a distributed feed line.
A multi slot configuration can be made according to the present invention, by having two slots excited either serially by the same feed line, e.g., crossing the first slot at its excitation point, continuing to second slot, crossing the second slot at its excitation point, and then having the load end part of the feed line. This embodiment enables the entire antenna to operate at the further frequency bands.
According to a further preferred embodiment, each of the slots may be excited by a separate feed line, the feed lines being in parallel to each other.
In another configuration according to the present invention, a further feed line may excite each of the two slots, while each of the feed lines constructed according to either the series or parallel methods as above-mentioned. It is to be appreciated that any combination of series and parallel feed lines may apply to the latter antenna according to the present invention.
The electrical connection to the antenna can be at any suitable point on the antenna. For example, plated through holes may be produced on the antenna PCB at a pre-design stage, and pins from the communication device""s PCB may be inserted into these holes and soldered. In another possible arrangement, spring loaded pins may produce the electrical connection by direct contact with pads on the PCBs of the antenna and the communication device. In a further possible arrangement, electromagnetic coupling between a feed line on the communication device""s PCB and the antenna can make the electrical connection to the antenna.
A preferred implementation is to have the antenna (or at least one of its layers, if more than one) an integral part of the communication device""s PCB. In the most general case, the device""s PCB is a multi-layer PCB, and the antenna can be easily produced directly on that PCB, thereby eliminating any need for any further connection or a separated PCB. The conductive reflector if applicable as a separate layer may then be a simple metal sheet placed close to the front cover of the device""s PCB, being electrically connected to the antenna, e.g. by conductive pins.
A further implementation is to have the upper layer of the device""s PCB a flexible layer, containing the antenna and the conductive reflector on it, in which either the ground panel or the conductive reflector panel is folded to produce the final antenna.
Another preferred embodiment is to have the antenna an integral part of the communication device""s battery, which is usually placed on the backside of the communication device. In such structure, the contact elements will preferably be of the type of spring-loaded pins. A preferred position to place the antenna is in the top of the back side of the communication device, in order to minimize interference with its operation and performance while holding the communication device in the user""s hands and/or near the user""s body/head.
It will thus be seen that the present invention may be implemented by an antenna comprised of a resonant slot (i.e., xe2x80x9cshort endedxe2x80x9d slot) cut in a ground plane of a printed circuit board, excited by at least one feed line crossing the slot at least at a single excitation point along the slot. This excitation point is designed to optimize the slot impedance to the feed line point at the desired operation frequency. The excitation may also be performed by a dual feed line, to excite the slot symmetrically to ensure symmetrical radiation of the slot, or asymmetrically to widen the frequency bandwidth of operation by a combination of two different excitations. In order to enhance the antenna efficiency, the load end side of the feed preferably is of a reactive type rather than a matched load. The design of the feed end of the feed line and the load end of the feed line may be made according to U.S. Pat. No. 5,068,670, to maximize the operational bandwidth of the antenna. The slot is preferably curved on the ground plane in which it was cut in, in order to ensure the small size of the antenna.
The load end is, as above-mentioned, of a reactive load type. It may be a shorted stub (simulating a short circuit, where the end of the stub is connected to the ground plane, e.g., by a plate-through-hole), and opened stub (simulating an open circuit), or lumped element/s (simulating a reactive load which may represent an impedance other than a short circuit or open circuit). Any combination of reactive loads may serve as the load end of the described antenna constructions.
As previously mentioned, modern mobile communication devices now require dual or triple band of operation. Therefore, the slot is designed to operate in the higher band/s (e.g., in the 1800 and/or 1900 MHz for cellular phone devices). In order for the antenna to operate also in the lower frequency band (e.g., in the 800 and/or 900 MHz for cellular phone devices), an extension of the ground plane may be produced at the far end of the slot by means of a sheet of metal electrically connected to the edge of the ground plane to add a further band of operation to the antenna (e.g. in the 800 and/or 900 MHz for cellular phone devices). The added piece of ground plane, together with the PCB of the mobile communication device, both tune the lower operational frequency band. Since the PCB of the communication device is pre-produced and in most cases is independent of the antenna design, the tuning is usually controlled by the shape, length, width and type of connection of the extended ground plane.
The above-mentioned extended ground plane may be applied on a PCB folded to the other side of the antenna""s PCB or as a second layer placed either at an angle, or parallel, to the antenna""s PCB in order to save surface of the antenna. In a preferred implementation, the ground plane extension is made by means of feed line stubs on the other side of the antenna""s PCB and electrically connected to the ground plane by plated through hole/s or conductive pin/s. These stubs are designed so they do not significantly interfere with either the feed/s and load/s of the feed line exciting the slot or the slot itself.
Further features and advantages of the invention will be apparent from the description below.